1. Field of the Invention
The present invention relates to systems, methods and devices for controlling the states of stimulating electrodes in body stimulating devices, such as cochlear implants and other neural stimulators.
2. Related Art
Electronic devices implanted within the body in order to stimulate nerve tissue (e.g. cochlear implants) for perceptual or functional purposes generally use platinum electrodes as the interface between the electronics and the body tissue. In general terms, such electrodes are selectively driven with a current in order to evoke a perception (for example sound) or a function (for example a limb movement) in the user. FIG. 1 illustrates this schematically. The platinum electrodes 101 are connected to the implant via insulated wires 104 and driven by the stimulating current 102, which passes through the tissue 100 and the nerve cell 107, and returns to the implant (return current 103). At the surface of the platinum electrodes 101, chemical reactions take place, changing the electron current in the electronics to an ion current 105 in the tissue; charge 106 remains on the electrode surface, causing an increase in voltage across the interface.
Under normal operation of the interface, these chemical reactions are reversible and when the current direction is changed, the reactions are reversed, leaving a neutral interface. It is usual for the stimuli to be structured as biphasic pulses, in such a way that there is no net charge delivered to the tissue. If, however, the current is allowed to flow in one direction for too long, toxic products can escape the interface and damage or destroy the surrounding tissue. Likewise, if the voltage across the interface is allowed to remain elevated for too long, toxic species are irreversibly generated at the interface. To ensure, then, that stimulation is safe, and that no toxic species escape the interface, it must be ensured that the DC and low-frequency (LF) states of the electrodes, i.e. the DC/LF interface voltages and the DC/LF interface currents, remain within certain bounds. The usual target values are some hundreds of milli-volts, or some tens of nano-amperes (for typical cochlear implant electrode areas of about 0.25 mm2).
The FDA in the US requires that the magnitude of the current through an electrode is below 100 nA measured over any 1 ms period. The use of charge-neutral pulses ensures, in principle, that the FDA requirement for the DC/LF current is met; in practice, however there will be a small error in the generated stimulation current. This requires a second measure to be taken to ensure low levels of DC/LF current at all times. This is particularly an issue when high stimulation rates and high current levels are used. Further, if the stimulation current source goes out of compliance, then significant charge errors can occur.
A number of approaches are currently employed to control the interface voltage and current.
One approach is to use DC blocking capacitors for each electrode to ensure zero DC currents through the electrodes. FIG. 2 shows such an arrangement, whereby for each stimulating electrode 201, there is a DC blocking capacitor 202 disposed in the path for stimulation currents 203. A capacitor may also be disposed on the monopolar return electrode 204. In order for this approach to be effective, it is necessary to provide a capacitor with relatively high capacitance, in the nano-farad range, for each electrode. With current capacitor technology, this cannot be fabricated in an integrated circuit, and so discrete components are required to be used. It is difficult to have a capacitor per electrode when tens of electrodes are in use, as this requires tens of capacitors, which significantly increases the required space for the implant. As the number of electrodes increase in future devices, potentially to hundreds or thousands, DC blocking capacitors become even more impractical. This type of approach is known from, for example, U.S. Pat. No. 5,324,316 to Schulman et al., U.S. Pat. No. 6,600,955 to Zierhofer et al., and U.S. Pat. No. 6,219,580 to Faltys et al.
Another approach is to use periodic short-circuiting of all electrodes to ensure that the DC/LF electrode voltage does not drift out of the safe window. FIG. 3 illustrates schematically such an arrangement, whereby a shorting switch 301 is provided for each electrode 201, and periodically the switches are closed. In some implementations, a series capacitor is used in the return electrode only. This approach allows for up-scaling of the number of electrodes. However, shorting all the electrodes requires the stimulation protocol to include an inactive period when no stimulation takes place. This approach is disclosed in European Patent No. 0,241,101 to Cochlear Limited.
Another approach is to measure the differential residual voltage between electrodes during a dead period and adjust the duration or amplitude of the applied stimuli to compensate for the charge error. This approach is disclosed in U.S. Pat. No. 5,674,264 to Cochlear Limited.
Using high-frequency asynchronous stimulation on many electrodes concurrently, and employing electrode arrays with a large number of electrodes, are considered by many as desirable in order to improve system performance in cochlear implants, and other implant systems. When concurrent, asynchronous stimulation is used, there is no dead period available to carry out electrode shorting.